In response to the increased need for higher bit rate and more efficient transmission of packet data over cellular networks, the WCDMA 3GPP Release 5 extended the WCDMA specification with the High Speed Downlink Packet Access (HSDPA), and Release 6 introduced Enhanced Dedicated Channel (E-DCH), often referred as Enhanced Uplink (EUL) or High Speed Uplink Packet Access (HSUPA). HSDPA and HSUPA together are called High Speed Packet Access (HSPA) which greatly improves the achievable bit rate over the air interface. 3GPP Release 7 introduced higher-order modulation and multiple input multiple output (MIMO) for HSDPA to further improve the achievable bit rate.
Similarly, a primary objective of a multi-carrier (MC) system is to achieve high data rate. A multi-carrier arrangement with frequency division duplex (FDD) can be described as a set of downlink carriers linked to a set of uplink carriers. The downlink carriers can be adjacent or non-adjacent in the frequency domain, and the same holds for the uplink carriers. Multi-carrier arrangements can also be used in time division duplex (TDD) systems. The component carriers in a multi-carrier system may also belong to different frequency bands. As one example, WCDMA/HSPA operating on multiple 5 MHz carrier frequencies is referred to as Multi-Carrier WCDMA or Multi-Carrier HSPA. In an E-UTRAN system, multiple component carriers such as four 20 MHz carriers in the downlink and two 20 MHz in the uplink (for FDD) can be used to enhance the data rate. So a multi-carrier system uses more than one carrier in the downlink and/or the uplink. One of the multi-carriers is called the primary or anchor carrier and the remaining one(s) is(are) called secondary or supplementary carriers.
The anchor carrier contains all physical channels including all common control channels. The secondary carriers may or may not contain all physical channels; for instance, they may lack some of the common downlink control channels. The anchor carrier in the downlink and in the uplink (i.e., if there is more than one carrier in uplink) should support legacy operation based on a single carrier, which means the downlink anchor carrier should contain all common channels so that the legacy single-carrier UEs are served. A multi-carrier UE also needs the anchor carrier to transmit all common control channels for acquisition of the frame timing, neighbor cell measurements, etc. Any single-carrier system can be evolved to a multi-carrier system to increase data rate. The future advancements of HSPA, E-UTRAN, and other systems will likely result in multiple carriers both in the uplink and the downlink, (e.g., 4 downlink carriers and 2 uplink carriers).
Different types of receivers exist. Some can receive multi-carrier transmissions—others cannot. Some receivers have interference cancellation capability—some do not. Even for those that do have interference cancellation capability, there are differences in those capabilities. For example, one type of receiver might be able to specifically cancel inter-cell interference, while another type of receiver cannot. The inter-cell interference is contributed by the signals transmitted from the neighboring cells.
In multi-carrier systems, although the anchor/primary carrier contains all the common channels, depending upon the network implementation, some of the common control channels may not be transmitted on the secondary carriers. The absence of a common control channel, like a synchronization channel, on the secondary carrier increases the complexity of the inter-cell interference cancellation in the receiver. Assume, for example, that a multi-carrier UE with an inter-cell interference cancellation receiver in a WCDMA type system is unable to use such a secondary synchronization channel (S-SCH) to identify the scrambling code group for an interfering neighbor cell. In order for the UE receiver to cancel inter-cell interference caused by interfering neighbor cells, the channel impulse responses of the interfering signals from each of the interfering cells must be determined. To perform this determination, the UE needs information about the master timing in each of the neighbor cells and the scrambling codes used in those neighbor cells. In the absence of synchronization signals on secondary carriers, the UE will have to extensively search to determine the scrambling codes used in the neighbor cells which drains UE power and slows down the synchronization process.
One approach to this problem might be to predefine that in multi-carrier systems, the UE enables the inter-cell interference cancellation only on the primary anchor carrier but not on the secondary carriers. But this approach leads to reduced performance on the secondary carrier because only non inter-cell interference cancellation receiver configurations may be used on the secondary carrier. If there are multiple secondary carriers, the performance loss will be higher.
Another approach is to signal the list of scrambling codes used on neighbor cells on the secondary carriers via some sort of signaling, e.g., RRC signaling. But this increases the signaling overhead and is difficult to include in legacy terminals and legacy versions of a specification like 3GPP. Furthermore, the neighbor cell measurements are made on cells belonging to the anchor carrier, which means that the signaling of an additional neighbor cell list for cells belonging to a second carrier would only be to assist inter-cell interference cancellation receiver operation on the secondary carrier. This extra network planning work to obtain a neighbor cell list solely for the inter-cell interference receiver may not be justified, and it would be preferable to avoid this extra work.
In summary, the network implementation determines whether all or a sub-set of common channels (e.g., SCH) are sent on the secondary carriers in a multi-carrier system. It is preferable that UEs perform inter-cell interference cancellation if possible, but in the absence of receiving the SCH on a secondary carrier, it is more demanding for the UE to perform inter-cell interference cancellation, and therefore, not necessarily desirable. On the other hand, if the SCH is available on a secondary carrier, then inter-cell interference cancellation can be performed at least with less complexity, power, and delay.